Fix implode() wrong parameter order (#6866)
It has been deprecated in PHP 7.4.
Such as PHP deprecated: implode(): Passing glue string after array is deprecated. Swap the parameters in /var/www/roundcubemail/program/lib/Roundcube/rcube_db.php on line 917
Signed-off-by: Jack Cherng <jfcherng@gmail.com>